Two different approaches are being used to develop better methods for the evaluation of the efficiency of anticancer drugs. (1) Rapid procedures have been employed to prepare important metabolic intermediates labeled with carbon-11 and fluorine-18. (11C)Thymidylate was used in animal experiments. We are now ready to carry out similar experiments with (18F)fluorouridylate. These compounds tagged with positron-emitting isotopes offer the potential of studying in vivo metabolism of DNA and RNA in normal and tumor bearing animals. (2) One-carbon flow into purines, thymidine nucleotides, and methionine is being investigated in synchronized cell cultures pulsed with labeled serine. We hope detailed information from about one-carbon metabolism at different times of the cell cycle will provide more definitive knowledge of the nature of growth regulation in normal and malignant cells.